Generation of frictional heat during machining processes, particularly during the high speed machining of metal, has been a problem for many years and is even more so in light of the desire for higher machining speeds in today's competitive market place.
Frictional heat build-up during the machining process can create a plethora of problems including distortion and degradation (including burning) of either or both the workpiece and the machining tool as well as significantly shorten tool life resulting in substantially increased costs associated with machining the particular part.
One machining process that is particularly prone to heat generation is grinding of which but one example is the grinding of vehicle differential ring gears that is hereinafter described in greater detail as an illustrative example.
Although a certain degree of control of heat build-up during the machining process can be accomplished by the use of petroleum and non-petroleum based metal working fluids hereinafter called coolants well known to those skilled in the art, such coolants have heretofor normally been used at or near ambient temperature and, when cooled, have been cooled down to near ambient temperature for re-use.
The only known instance of attempting to cool a workpiece substantially below ambient temperature is where pressurized carbon dioxide in the liquid phase has been injected into a water stream to produce a mixture of water and ice particles that is then directed onto the workpiece being machined such as disclosed in U.S. Pat. No. 4,809,859, the disclosure of which is incorporated by reference. Although such may in certain cases be used to advantage, the ice and water mixture would not possess attractive lubricity characteristics; would not possess progressively increasing lubricity characteristics with decreasing temperature; and could conceivably lead to corrosion of the workpiece being machined especially ferrous alloys.
Others have attempted to inject the coolant close to the location at which the machining tool engages the workpiece to enhance cooling efficiency such as disclosed in U.S. Pat. No. 4,621,547 where the coolant is directed at high velocity at the region of the workpiece being machined and U.S. Pat. 4,695,208 where the machining tool includes fluid channels for conveying the coolant to the point of engagement with the workpiece, the disclosures both of which are included herein by reference.
In contrast, the present invention eliminates the problems associated with the water and ice mixture previously described by cooling temperature responsive coolants to below ambient temperature to provide a consistency that greatly enhances its lubricating ability enabling substantially higher machining speeds while minimizing frictional heat build up during the machining process and enhancing conductive heat transfer into the machining tool.
In many instances the preferred coolant consistency may be virtually a solid-like film to provide the degree of lubricity desired to minimize frictional heat generation during high speed machining operations. Although paste lubricants have been used in the past for certain machining operations, such pastes have commonly been difficult to apply; have characteristically been unable to be re-used because of inability to separate machined particles and other debris accumulated during the machining operation; and are difficult to apply in a repeatably uniform manner.
In contrast, the cooled coolants of the present invention are able to be applied separately or together, commonly at ambient temperature as fluid/liquids or gas/liquids and, as such, can be applied with a high degree of uniform repeatability and, upon returning to ambient temperature, can be cleaned (such as by filtering) for re-use.